correct plumbing for radiant connection to heat loop

Hi all, I'm a new homeowner dealing with a hydronic heat system that services both radiant heat downstairs (trays installed over subfloor with wooden flooring on top) and forced air upstairs. The radiant floors are divided into three zones, with a single pump and zone valves controlling water delivery there. The boiler is an on-demand Munchkin type.
We've been having some trouble with the system since moving in. I'm learning about these systems as I go so please do question or correct me.

Initially, the radiant downstairs was working reasonably though some spots in the home had trouble coming up to set temps. However the upstairs system had zero heat when activated, and the supply and return pipes at the air handler remained cold. Previous owners did not run the upstairs heat often or at all it turns out. A service company came out and bled some air out of the water loop running up to the attic unit.

This got the upstairs unit pumping some hot water, but led to a second problem: the system would heat until all zones shut down, then the pressure in the system would drop close to zero, which in turn caused the boiler to shut down. Adding a little make-up water would bring the pressure back-up, and the cycle would repeat. Another new thing: psi would swing around a lot more, and hit 30 psi at the top end.

As I continued researching how these things work, I realized that there was no expansion tank installed. As I thought about this my going theory was that excessive air in the upstairs had allowed expansion previously, and now that we had eliminated it we were hitting excessive pressure levels and tripping the PRV in the boiler at 30 psi. A cup under the relief pipe proved that we were in fact losing water (about 2 cups with every cycle I described above).

I asked the service company about this and they contended that the radiant system PEX tubing should accommodate the expansion and that a tank wasn't necessary. This seems to contradict just about everything I can find online on the subject. The problem continued - and on their next visit the service company installed one without my asking them to do so. Now pressure in the system stays at approx. ~20 regardless of activity, without adding make up water.

But the latest problem: we were away when the tank was added, and came back to the house after about a week. The tstats had been left at 50, we walked into the home that was about 45 degrees or so. I didn’t think about it and turned up the thermostats. After running for about two hours we realized nothing was happening. Pressure was correct, boiler was running. Feeling around on various pipes in boiler closet no heat was detectable on the supply side of the radiant system at all, but the heat loop off the boiler was hot as it should have been. The two pumps in that closet were running. The boiler was not cycling much, tell me that not much cold water was coming in.

The next day I attempted to bleed and refill the radiant zones. I didn’t hit any noticeable air bubbles from the downstairs pipes. I purged the upstairs loop, and here I did see a lot of air come out. I’m assuming and hoping this came in while the expansion tank was being installed. Thinking I had the problem licked, I let the system run, but same issue.

I started to wonder next about the piping. The radiant side has a bypass between supply and return below the heat loop connection, and I started thinking that it was just pumping back its own return water. It had not seemed to do this before, but when the service company added the expansion tank, they tee’d it off the supply line for the radiant loop - so this was a change (more on that later).

Pursuing this, I started partially closing the valves on the radiant return lines to see if I could restrict the water flowing back in, and with a delicate adjustment suddenly hot water starting coming out of the heat loop and into the radiant. Open the return valves back up, the supply pipes got cold again, kind of proving my theory above. Left restricted, the floors very gradually warmed up.

My only hunch as to why this thing started behaving differently regarding water flow through the radiant: the expansion tank. Everywhere I can see references for these tanks, they are always shown on the return side, usually off the make-up water line, never on the supply side, where ours seems to be at point A. Is this a problem? My service company says no.

From every example I could find of radiant systems, the supply and returns sides generally never directly connect below the heat loop connection (point B), except maybe for a differential pressure bypass, but instead we have an open line (point C). Is this a problem? Should the connection at point B be eliminated, essentially extending the heat loop through to point C? Again, my service company assures me how it is now is how it should be piped. Note that the same company recently repiped the heat loop side when the previous boiler setup had failed, prior to my purchasing the house. I'm wondering if even when it worked it might not have been working as efficiently as it could have.

There is a manual purge valve upstairs but no air bleeds/eliminators. Given that this is the topmost point in the system, seems like a natural place for air to want to accumulate, should there not be some mechanism to trap/release air up there?

I've read a lot of posts here.. the tstats in the house are all standard. Given my setup, is lag a problem and should I consider a different thermostat setup downstairs?

Thanks in advance, this is my first question, but the folks who frequent this forum with answers have helped me learn a bunch already.

Based on drawing and there is better ways to do it, your missing a circulator to pull heat from the boiler primary loop to feed the radiant return manifold. Locate this on the lower pipe Pumping right to left, for good heat transfer via reverse flow. Wire with radiant circuit so they run together.

No picture, but it seems you need a "good" plumber/heating person who knows what he is doing to service it. depending on PEX expansion to perform the task of an expansion tank, implies a lack of experience or knowledge. Any good service person who performed any task on a heating system would have bled all air out and made sure it was working before he left.

Based on drawing and there is better ways to do it, your missing a circulator to pull heat from the boiler primary loop to feed the radiant return manifold. Locate this on the lower pipe Pumping right to left, for good heat transfer via reverse flow. Wire with radiant circuit so they run together.

Click to expand...

Thanks for the reply, by the lower pipe do you mean the pipe from the first tee, or the pipe segment on the bottom left of my diagram? There are two circulators in there where I labeled them "Taco", not sure I used the right symbol and sorry if i wasn't clear, but both are pumping right to left, one is in the heat loop (with an IFC), the other in the radiant supply side (no IFC). Did you mean something besides these two?